1. Field of the Invention
The present invention relates to a muffler for a compact combustion engine used as a drive source in a portable work machine such as, for example, a brush cutting machine or a cleaning blower.
2. Description of the Prior Art
In this type of engine, it is generally desired to allow the combustion engine to discharge exhaust gases from an exhaust outlet to the outside after the temperature of the exhaust gases has been reduced.
On the other hand, in recent years even the compact combustion engines have come to use an oxidization catalyst for substantially purifying the exhaust gases, specifically reducing respective concentrations of HC and CO contained in the exhaust gases. The oxidization catalyst referred to above is incorporated in a muffler to allow unburned components of the exhaust gases to be reburned in contact with the catalyst. As is well known, reburning of the exhaust gases in contact with the catalyst results in increase of the temperature of the exhaust gas discharged from a discharge port. Accordingly, attempts have hitherto been made to use an auxiliary casing, formed separate from the muffler, on an exhaust side of the muffler so that the exhaust gases when flowing through the auxiliary casing can be cooled and then discharged to the outside. See, for example, the Japanese Laid-open Patent Publication No. 2002-242666, published Aug. 28, 2002.
It has, however, been found that the mere use of the auxiliary casing in association with the muffler incorporating a catalytic converter therein, such as disclosed in the above mentioned patent publication is incapable of providing a sufficient cooling effect since the temperature of the exhaust gases is extremely high. Also, while the wavelength of the pulsating motion of the exhaust gases tends to increase with decrease of the number of revolutions of the combustion engine and the velocity of flow of the exhaust gases is hence lowered, the distance from an outlet of the catalytic converter to the discharge port is short in the above mentioned patent publication, and as consequence, air tends to be sucked from the discharge port by the action of the pulsating motion of the exhaust gases to flow, in reverse to the outgoing exhaust gases, into a high temperature chamber after having passed through the catalytic converter, resulting in contact with high temperature unburned components of the exhaust gases that are left without being reburned in contact with the catalyst.
Where the separate auxiliary casing is fluidly connected with the exhaust side of the muffler by the use of connecting elements, such as disclosed in the above mentioned patent publication, the number of component parts used and the number of fitting steps tend to increase, resulting in increase of the cost.